The hypothesis is that coumarin can be used as a probe to study cytochrome P450 2A in humans and its regulation by exogenous factors such as drugs. It is also hypothesized that cimetidine will inhibit coumarin 7-hydroxylation in humans. Twelve men will receive high-protein and high-carbohydrate diets each for 15 days. Cimetidine will be given on Days 10-15 and theophylline metabolism will be studied on Days 8 and 15.